1. Field of the Invention
The present invention relates to a Ni-based superalloy, and a cast product and gas turbine rotor and stator blades using the Ni-based superalloy.
2. Background Art
Recently, an increase in thermal efficiency in an internal combustion engine has been tried, from the viewpoint of growing environmental consciousness such as the saving of fossil fuels, reduction in carbon dioxide emissions and prevention of global warming. It is known that a thermal efficiency can be most effectively enhanced by operating a high temperature side of Carnot cycle at a higher temperature in a thermal engine such as a gas turbine and a jet engine. In accordance with a higher turbine inlet temperature, an importance of an improvement and development of materials used as hot parts of a gas turbine, i.e., a combustor or turbine rotor and stator blades, is enhanced. In order to deal with such a higher turbine inlet temperature, a Ni-based heat resistant superalloy having a better high-temperature strength is applied as a material, and many Ni-based superalloys are used at present. Examples of the Ni-based superalloy include a conventional casting superalloy having an isometric crystal, a directionally solidified superalloy having a columnar crystal, and a monocrystal superalloy having one crystal. In order to enhance the strength of the Ni-based superalloy, it is important to add Al and Ti and the like to precipitate many γ′Ni3(Al, Ti) phase, which is a reinforcing phase, together with adding many amounts of a solid-solution reinforcing element such as W, Mo, Ta and Co.
In the meantime, due to an elevated fuel price, a low quality fuel containing a large amount of impurities causing a corrosion starts to be used as a fuel for an industrial gas turbine, and it is also necessary to develop a material having both high-temperature strength and corrosion resistance. In such a material, it is desirable that a large amount of Cr forming a protective coating film be added. Examples of a superalloy, which gives weight to corrosion resistance, include conventional casting superalloys which are disclosed in, for example, JP Patent Publication (Kokai) No. 2004-197131 A, JP Patent Publication (Kokai) No. 51-34819 A (1976) and JP Patent Publication (Kokai) No. 2010-84166 A.
However, problem of such a superalloy are: when many amounts of elements added are contained, the stability of the constitution of a material is further lowered and a hard and brittle harmful phase such as a δ phase is precipitated during a long period time of use. That is, it is difficult to develop a superalloy material having a good high-temperature creep strength and simultaneously corrosion resistance and oxidation resistance.